Boötes, the Herdsman

Plus the planets for June.

Did you know there is a kite in our sky? No, not one at the end of a string, but one made of stars. The constellation Boötes, the Herdsman, is shaped like a kite. The base of the kite is the fourth-brightest star in the sky, Arcturus. Working up the kite-shape, the lower sides are formed by Epsilon Boötis on the east and a pair of stars on the west, Sigma Boötis and Rho Boötis. Farther up the kite, the crossbar is formed by Delta Boötis to the east and Seginus (Gamma Boötis) to the west. The top of the kite is Nekkar (Beta Boötis). All these stars are at least fourth magnitude, so they are pretty obvious from a moderately dark sky.

Click the image for a larger map.

Facing south and looking overhead you will find a kite-shaped constellation overhead, Boötes, the Herdsman. NGC 5248 in the southwestern part of the constellation is a member of the Virgo cluster. You can follow the arc of the handle of the Big Dipper southward to Arcturus in Boötes and follow that arc onward to Spica in Virgo (just off the southern edge of this chart).

One easy way to identify Boötes's brightest star, Arcturus, is to start at last month's constellation, Ursa Major. Follow the arc of Ursa Major's tail southward, away from the bowl of the dipper, and you will run right into Arcturus. If you continue onward in the same arc, you will hit the first-magnitude star Spica in the constellation of Virgo. This can easily be remembered by the phrase, "arc to Arcturus and speed on to Spica."

The mythology behind Boötes is a little murky. It was first mentioned by Homer in the Odyssey as a navigational reference point. There are multiple stories, identifying Boötes as a herdsman, ox driver or bear driver. A different story had Boötes representing Icarius, a grape grower in Attica (not to be confused with Icarus, who flew too close to the Sun). Icarius was a friendly guy who let the disguised Bacchus, the god of wine and intoxication, inspect his vineyards. Bacchus was so impressed that he taught Icarius how to make wine.

Icarius invited some local shepherds to sample the new wine. They all drank too much and eventually fell asleep. When they awoke, they decided Icarius had tried to poison them and killed the still-sleeping Icarius, throwing his body in a ditch. His dogs started howling and Erigone, his daughter, released them to search out Icarius's body. Filled with grief, Erigone killed herself and the two dogs jumped into the ditch and died as well. The gods put all of them in the sky, Icarius as Boötes, the dogs as Canes Venatici, and Erigone as Virgo.

Arcturus, the brightest star in Boötes at magnitude -0.04, is a red giant star only 36.7 light-years away. It is 170 times brighter than our Sun, but it is much redder — type K1.5 IIIpe. The "pe" means its spectrum contains "peculiar emissions" in the form of bright emission lines from various elements in its relatively cool atmosphere. Arcturus is the brightest K-type star in the sky and was the first star (besides the Sun) to be observed in the daytime with a telescope, in 1635 by French mathematician and astronomer Jean-Baptiste Morin.

When we look toward Boötes, we are looking out of the plane of our galaxy, so it contains few nebulae, open or globular clusters. A number of faint galaxies can be seen, far beyond our Milky Way, including NGC 5248, a 10.2-magnitude spiral galaxy way down at Boötes' southwestern corner. This galaxy is fairly small, 6.1 x 4.4 minutes of arc across. Measurements of its distance vary from 41.4 to 74.0 million light-years away, but it is usually reported as 59 million light-years distant.

NGC 5248 is a member of the Virgo supercluster of galaxies. Early astronomers discovered there were many more galaxies in and around the constellation Virgo than in other parts of the sky. It was not clear at first whether this was an actual cluster or just a chance alignment of galaxies. The argument continued into the 1970s when the first large-scale surveys of galactic red-shift were performed. This measurement of the displacement of emission lines in their spectra, caused by the expansion of the universe, provided a measurement of the distance to each galaxy. These measurements proved conclusively that Virgo supercluster galaxies, including NGC 5248 in Boötes, were roughly the same distance away. That means they are actually close together in space.

The Planets for June

The first week of June, you can find all the naked-eye planets except Venus in the evening sky. If you sweep along the ecliptic from west to east, you will find Mercury, Jupiter, Mars and Saturn. Venus alone is only in our morning sky.

Mercury is low on the western horizon as June begins. It was best viewed last month for this apparition, but you can still find it in the evening for the first week of the month. On June 1, Mercury will be magnitude +1.5 with a disc 9.7 seconds-of-arc across. It will be a crescent that is only 20.6% illuminated, becoming thinner until it is no longer observable. Mercury will swing between the Earth and Sun on June 19. The Messenger of the Gods will move into the morning sky next month.

Jupiter is moving eastward in eastern Gemini, only 17 degrees up in the west as it gets dark and setting around 10 p.m. At midmonth, the King of the Gods is magnitude -1.9 with a disc 30.1 seconds-of-arc across.

This month, Mars sets around 2 a.m. You can find it 50 degrees up in the south as it gets dark. At midmonth the God of War shines at magnitude -0.2 with a disc 10.5 seconds-of-arc across. Mars is moving eastward in central Virgo.

Farther east, Saturn is just past opposition, so it is a great time for a telescopic look. Saturn is moving slowly westward in central Libra, 35 degrees up in the southeast as it gets dark and setting around 4 a.m. Saturn's Rings are 41.3 seconds-of-arc across and tilted down 21.2 degrees with the northern face showing. Its disc is 18.2 seconds-of-arc across.

Venus starts the month in western Aries. It travels eastward through Aries and into central Taurus, ending the month near the Hyades star cluster. It rises around 4 a.m. in the east. The Goddess of Love's disc is 12.8 seconds-of-arc across and 81% illuminated. Venus is heading back around the far side of the Sun, making its disc fuller each day.

June 21 at 4:51 a.m. marks the beginning of summer in the northern hemisphere with the June Solstice. This is the instant the Sun reaches its northernmost point in the sky each year, marking the longest day and shortest night. After this date, the Sun begins to move back southward. So make the best of the little darkness you have this month and "keep watching the sky"!

An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.